Keywords

1 Introduction

The proton exchange membrane fuel cell (PEMFC) is an energy conversion device that converts the chemical energy in hydrogen into electrical energy. It has the advantages of small size, light weight, fast start-up, high energy efficiency, and the generation of only water, making it an ideal power source for zero-emission vehicles (ZEV) [1]. However, high cost is the key factor to limit the industrialization and commercialization of PEMFCs, and society still prefers diesel locomotives to fuel cell vehicles before reducing the price of fuel cells. Current analysis of the cost structure of PEMFCs shows that in high-volume production, the proton exchange membrane (PEM) accounts for 11% of the stack cost and the platinum catalyst (Pt/C) accounts for 49% of the total cost [2]. In order to realize the commercialization of PEMFCs, it is necessary to reduce the material costs, including reducing the use of Pt catalysts, and seeking cheap catalysts, bipolar plates and proton exchange membranes. Therefore, the development of low platinum catalysts or non-noble metal catalysts has attracted increasing attention.

Pt/C is the catalyst commonly used in PEMFCs. This metal is selected because platinum is a relatively stable material, which can effectively accelerate the oxygen reduction reaction (ORR) at the cathode. Platinum is both expensive and scarce, with a price of $1500 per troy ounce [2]. More and more researches are devoted to reducing the content of platinum in fuel cell. One approach is to use platinum catalysts with different material carriers to enhance their performance, dispersibility and durability [3, 4]. These platinum carriers can be carbon particles, carbon nanotube particles, graphene and metal particles with a non-platinum metal core and a platinum shell structure. Another approach is to alloy platinum with cheaper metals such as palladium (Pd), ruthenium (Ru) and especially silver (Ag) [5, 6]. Recent studies even suggest that platinum may be completely replaced by non-platinum catalysts, which are low in cost, rich in raw materials, easy to synthesize and high in oxygen reduction performance [7, 8]. More and more scholars are studying and preparing high-performance non-noble metal catalysts. The common non-precious catalysts include non-precious metal sulfides, non-precious metal carbonitrides, non-precious metal oxides, non-precious metal nitride, non-precious metal nitrogen oxides, M-N/C catalysts, etc. Among them, M-N/C catalysts are the most likely non-precious metal catalysts to replace platinum catalysts [9].

The non-precious metal catalyst Fe-N-S-C can be used as an ORR catalyst for the preparation of the cathode catalyst layer (CL) of fuel cells. In this paper, the cathode CL is prepared by Fe-N-S-C catalyst, and the anode CL is prepared by low-load Pt/C catalyst. The prepared membrane electrode assembly (MEA) is assembled into fuel cells and its performance is tested. The effects of the Fe-N-S-catalyst loading and the ratio of Nafion ionomer to Fe-N-S-C catalyst on the performance of fuel cell are studied, and the effects of inlet pressure, operating temperature and inlet humidity on the performance of MEA of non-noble metal catalyst are investigated. The electrochemical impedance spectra (EIS) of the fuel cell at different humidity and different current loading are tested for reaction kinetics analysis. And the durability of the non-precious metal catalyst MEA is studied.

2 Experimental

2.1 Preparation of Fuel Cell

The preparation process of Fe-N-S-C catalyst has been described in detail in our previous work [9]. The non-noble metal catalyst Fe-N-S-C used to prepare the cathode CL of the fuel cell is prepared, and the anode CL is prepared using the low-load Pt/C (Pt 60 wt%, Johnson Matthey) catalyst. The Pt loading of the anode CL is 0.1 mg cm−2, and the mass fraction of Nafion (5 wt%, Dupont) ionomer is 25 wt%. In order to investigate the optimal loading of Fe-N-S-C catalyst and the optimal ratio of Nafion ionomer to Fe-N-S-C catalyst, several MEAs are prepared for comparison. The non-precious metal catalyst MEAs used in this paper are all home-made in the laboratory. The two kinds of catalysts ink are sprayed on the Nafion 212 membrane, and then the catalyst coat membrane (CCM) and gas diffusion layer (GDL) are hot pressed at a constant temperature of 130 ℃, a pressure of 500 psi and a hot-pressing time of 2 min. The reaction area of the fuel cell is 2 cm × 2 cm. The cells are assembled in order and the bolts are tightened in diagonal order with a torque wrench with a torque size of 5–7 N m.

2.2 Performance Evaluation and Electrochemical Measurements

The assembled fuel cell is connected to a fuel cell testing system, which includes a control system, gas supply system, a humidifying system and back pressure system. Firstly, before testing the polarization curve, the fuel cell is activated to improve the utilization ratio of the catalyst, and the membrane is humidified to establish the transmission channels of protons, electrons, reactants and products. The activation process of fuel cells generally adopts natural activation and forced activation, and the constant current forced activation is adopted in this experiment. When testing the polarization curves, the average value of each group of experiments is taken after three repetitions. EIS tests are carried out in the frequency range of 0.1 Hz–1 kHz with 5% loading current amplitude, and each EIS test is repeated three times.

3 Results and Discussion

3.1 Effect of Fe-N-S-C Loading on Cell Performance

The fuel cell performance is directly related to the cathode catalyst loading, and the effect of Fe-N-S-C catalyst loading on the fuel cell performance is explored here. The Pt loading of the anode CL is controlled to be 0.1 mg cm−2, and the Fe-N-S-C catalyst loading of the cathode CL is 0.5 mg cm−2, 1.0 mg cm−2, 2.0 mg cm−2, and 3.0 mg cm−2, respectively. The fuel cell is operated at 70 ℃, the relative humidity (RH) is 100%, and the hydrogen and oxygen flow rates were 100 and 200 mL min−1, and the back pressure is 50 kPa. The current density-voltage curves and current density-power curves of the MEAs are shown in Fig. 1, and the maximum power density of each MEA is shown in Table 1.

It can be seen from the experimental results that when the loading of Fe-N-S-C catalyst is less than 2.0 mg cm−2, the performance of the fuel cell will be improved with the increase of the loading, because more catalysts are involved in the ORR, which improves the electrochemical reaction rate, thus improving the energy density of the fuel cell. The cathode CL Fe-N-S-C with the loading of 2.0 mg cm−2 has obtained the fuel cell with the highest power density and the best performance. However, with the increasing load of non-precious metal catalyst, the performance of fuel cell does not increase, but decreases. Previous studies show that this Fe-N-S-C catalyst has a high specific surface area and a rich pore structure, and the thickness of the CL increases significantly with the increase of catalyst loading. When the catalyst loading exceeds 2.0 mg cm−2, the thickness of the Fe-N-S-C CL exceeds 45 μm. Too thick CL will increase the internal resistance of the MEA, and hinder the transmission of reactants and products, leading to the decline of fuel cell performance. Therefore, the optimal loading capacity of Fe-N-S-C catalyst is 2.0 mg cm−2, and this optimal loading capacity is selected for all the experiments.

Fig. 1.
figure 1

Polarization curves of MEA with various Fe-N-S-C content at 70 oC and 100 mL min−1 of H2 and 200 mL min−1 of O2 flow rates under 100% RH, and gas pressure is 50 kPa.

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Table 1. Maximum power density of MEA with different Fe-N-S-C catalyst loadings.
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3.2 Effect of Ionomer and Catalyst Ratios on Cell Performance

The influence of the ionomer loading on the fuel cell performance is very important. In this paper, the ratios of ionomer to Fe-N-S-C catalyst is 3:2, 3:3, 3:5 and 3:7, respectively. The performance of the prepared MEA is shown in Fig. 2, among which the ratio with the best cell performance is 3:7, and the power density of the fuel cell is 283 mW cm−2, which is much higher than that of the MEA with other ratios, and the performance of several groups of MEA with higher ionomer loadings is poor.

On the one hand, the ionomer in the CL serves as an adhesive and provides a transport channel for hydrogen ions in the CL. The appropriate amount of ionomer in the CL is beneficial to form a good hydrogen ion transport channel, improve the utilization of the catalyst, and enhance the performance of the fuel cell. The proper ratio of ionomer to catalyst is beneficial to improve the performance of the membrane electrode. Excessive ionomer will severely block the pore structure of the catalyst surface, which will prevent the transfer of the reaction gas to the active site of the catalyst and reduce the rate of ORR, thus reducing the performance of the fuel cell. In addition, excess ionomer will retain the water generated from the cathode reaction in the CL, causing flooding, impeding the transport of the reaction gas and increasing the internal resistance of mass transfer.

Fig. 2.
figure 2

Polarization curves of MEA with various mass ratio of Nafion and Fe-N-S-C catalysts at 70 oC and 100 mL min−1 of H2 and 200 mL min−1 of O2 flow rates under 100% RH, and gas pressure is 50 kPa.

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3.3 Effect of Relative Humidity on Cell Performance

To investigate the cell performance of the non-precious metal catalyst at different RH, the polarization curves of the MEAs are tested at the inlet RH of 100, 80, 60, 40 and 20% for both cathode and anode, where the cathode Fe-N-S-C catalyst loading is 2.0 mg cm−2 and the anode Pt loading was 0.1 mg cm−2, the operating temperature is 70 ℃, the back pressure is 50 kPa, and the polarization curves of the non-precious metal film electrode at different RH are shown in Fig. 3.

From Fig. 3, it can be seen that the performance of the MEAs gradually improves as the RH of the inlet gas decreases. When the RH is 100%, the maximum power density of the membrane electrode is 217 mW cm−2, when the RH drops to 40%, the maximum power density of the MEAs is 246 mW cm−2, and when the RH of the inlet air continues to decrease, the power density drops slightly to 241 mW cm−2. For the MEA which does not have self-humidification ability, it faces a sharp decrease or even loss of proton conduction ability under low humidity conditions. The experimental results show that the MEA with Fe-N-S-C catalyst as the cathode CL has a certain self-humidifying capability, which can ensure good performance under low humidity conditions. The self-humidifying performance of this MEA is related to the high specific surface area and abundant porosity of the Fe-N-S-C catalyst, and the specific reasons and self-humidifying performance have been explained in our previous work [10].

Fig. 3.
figure 3

Polarization curves of MEA under various RH (100, 80, 60, 40 and 20%) at 70 oC and 100 mL min−1 of H2 and 200 mL min−1 of O2 flow rates, and gas pressure is 50 kPa.

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In order to further analyze and understand the electrochemical performance of the MEA at different RH, the EIS of the fuel cell is tested. The test results are shown in Figs. 4, and 5 shows the changes of activation loss, ohmic loss and concentration loss with time. The EIS is carried out when the current density of the fuel cell is 30 mA cm−2, 300 mA cm−2and 1000 mA cm−2, respectively.

Comparing the EIS of the fuel cell at a current density of 30 mA cm−2, it can be found that the ohmic resistance and charge transfer resistance of the fuel cell increase with the decrease of the RH of the fuel cell, because the cathode reaction rate is slow when the fuel cell is running at low current, and it can not generate enough water to wet the MEA. When the RH is 20%, the anode charge transfer resistance and cathode charge transfer resistance can be measured, indicating that the membrane electrode is extremely dehydrated and the proton conductivity is very low.

Fig. 4.
figure 4

Impedance spectra at 30, 300 and 1000 mA cm−2 of MEA at 70 oC and 100 mL min−1 of H2 and 200 mL min−1 of O2 flow rates under (a) 100%, (b) 80%, (c) 60%, (d) 40% and (e) 20% RH, and gas pressure is 50 kPa.

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However, at a current density of 300 mA cm−2, the resistance of each part of the MEA did not increase significantly, thanks to the self-humidifying ability of the Fe-N-S-C catalyst, which preserves the water from the gas supply and the water generated by the ORR, and when the membrane and the anode CL are water-starved, the water from the cathode is transferred to the anode through the diffusion effect generated by the water concentration gradient, thus ensuring the proton conduction ability of the membrane and ionomer.

When the cell is operated at high current density, the EIS can detect the mass transfer resistance of the MEA, and it can be seen from the figure that the ohmic resistance and charge transfer resistance of the fuel cell operating at 1000 mA cm−2 do not change significantly, but the mass transfer resistance of the MEA is the largest at a RH of 100%, which may be due to the water retention effect of the Fe-N-S-C CL that makes the MEA difficult to remove water in time under high humidity operating conditions, which causes flooding and hinders the reaction gas transfer of the fuel cell.

Fig. 5.
figure 5

Impedance spectra at (a) 30 mA cm−2, (b) 300 mA cm−2 and (c) 1000 mA cm−2 of MEA.

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3.4 Effect of Inlet Pressure on Cell Performance

Generally, under the high current operating conditions, the voltage loss of the PEMFC is mainly caused by the transmission loss of reaction gas, and the performance of fuel cell can be improved by increasing the pressure of the reaction gas appropriately. The results of this study about the membrane electrode of non-precious metal catalyst are shown in Fig. 6.

The Fe-N-S-C loading of the membrane electrode is 2 mg cm−2 the Pt loading of the anode is 0.1 mg cm−2. As can be seen from Fig. 6, the back pressure of the reaction gas has a very significant influence on the performance of the fuel cell. The maximum power density of the membrane electrode was 142 mW cm−2 when no back pressure was applied to the reaction gas, and as the back pressure of the reaction gas increased to 50 kPa, the maximum power density of the membrane electrode rose to 191 mW cm−2, which was enhanced by 25%. When the back pressure of the reaction gas continued to increase to 100 kPa, the power density continued to rise by 17% to 223 mW cm−2 at 50 kPa. The experimental results showed that for this MEA with non-precious metal catalyst had a higher mass transfer resistance and exhibited poorer performance without increasing the back pressure. This is because the Fe-N-S-C catalyst is porous and the CL is thicker, which hinders the transmission of the reaction gas to a certain extent.

Fig. 6.
figure 6

Polarization curves of MEA under various gas pressure at 70 oC and 100 mL min−1 of H2 and 200 mL min−1 of O2 flow rates.

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3.5 Durability of Fuel Cell with Non-precious Metal Catalyst

In the durability test of MEA with non-precious metal catalyst, its durability is not ideal, and voltage decays quickly in a short steady-state test. In this experiment, the voltage decay of MEA is studied over 50 h when operating at a current density of 200 mA cm−2, and the experimental results are shown in Fig. 7. The voltage of the cell decayed by 40% in the first 30 h and remained basically stable in the next 20 h. The decay may be due to carbon corrosion, loss of catalytic active sites or oxidation of nitrogen atoms in the catalyst by hydrogen peroxide. Compared with the durability of MEA with noble metal catalysts, the durability of MEA with non-precious metal catalysts is much worse, and there is still much work to be done to improve the stability and corrosion resistance of the non-precious metal catalysts themselves.

Fig. 7.
figure 7

Long-term discharge curves of MEA operated at 70 ℃ and 100% RH. The hydrogen and oxygen flow rates are 100 mL min−1 and 200 mL min−1, respectively, and gas pressure is 50 kPa.

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4 Conclusions

In this paper, the MEA with Fe-N-S-C catalyst is successfully prepared. When the loading of Fe-N-S-C catalyst is 2.0 mg cm−2 and the ratio of Fe-N-S-C catalyst to Nafion ionomer is 7:3, the performance of MEA is the best. The performance of MEA under different RH conditions is tested, and it is found that it could still maintain good performance under low humidity conditions and has certain self-humidification ability. The influence of inlet pressure on the performance of MEA is studied. It is found that the performance of MEA is obviously improved with the increase of inlet pressure. The 50-h durability of fuel cell is tested at a current of 200 mA cm−2. The voltage of the fuel cell decreased by 40% in the first 30 h and remained basically stable in the next 20 h. This shows that the durability of this Fe-N-S-C catalyst needs to be greatly improved.